The Sanya Incoherent Scatter Radar (SYISR) is an advanced phased array radar system whose main task is to make observation and research of the Earth's ionosphere. SYISR is in a low latitude area, where many spacecrafts fly over. The existence of these hard target causes the echo signal several times stronger than the scattered signal by the ionosphere, which significantly influences ionospheric parameters retrieval. To effectively eliminate the hard target signal from the original data, this paper adopts the idea of constant false alarm rate (CFAR) to detect and eliminate the hard target. Since the background ionosphere is not stable, an adaptive VI-OSCFAR algorithm based on the traditional OS-CFAR is proposed. In this article, we compare the new algorithm with the traditional four algorithms and apply it to the realistic SYISR data inversion. The preliminary results show promising validation of the method, which will be potentially used in radar operation in the future.

本文针对13位巴克码匹配滤波器会因旁瓣导致地基雷达月球成像质量恶化的问题,提出了使用无旁瓣滤波器解码来避免出现旁瓣从而改善成像质量的方案,并用三亚非相干散射雷达进行了月球成像实验,检验了该方案有效性.成像算法采用了距离-多普勒算法.成像结果表明,13位巴克码匹配滤波器的旁瓣会使距离-多普勒图像在距离向产生10％～30％的回波强度扩散,污染周围距离门,导致距离分辨率恶化;而无旁瓣滤波器可以有效避免这种污染,从而优化距离分辨率,使成像质量得到改善.

When ground-based radar and range-Doppler algorithm are used to image the nearside of the Moon, it is inevitable to encounter the problem of "north-south ambiguity". This is because when the range-Doppler imaging algorithm is used to image a rotating celestial body, the echoes of the two points conjugated at the apparent equator are superposed together and cannot be resolved in the range-Doppler image. We propose a solution to this problem based on the Sanya Incoherent Scatter Radar, namely the mosaic imaging technology of the northern and southern hemispheres of the near side of the Moon. In this technology, two independent experiments were carried out to separately illuminate the northern and southern hemispheres of the nearside of the Moon by adjusting the beam direction to a specific position. Finally, a complete Moon nearside map was obtained by combining the images of the northern and southern hemispheres. The results of experiments show that this technique can successfully get the Moon images, but there are still some defects that need to be improved.

历经25年的发展,基于GNSS的无线电掩星技术已经成为一种有效的临近空间探测手段,但是在无线电大气掩星观测及数据处理过程中不可避免地会受到各种误差的影响,其中,电离层是重要误差源之一.由于电离层小尺度因素的存在,GNSS信号在穿越电离层过程中会受到影响,表现为振幅和相位上的振荡,并最终影响反演所得的大气弯曲角产品.文章基于COSMIC大气弯曲角数据,研究电离层小尺度因素中扩展E层(Es层)及F层不均匀体在中低纬地区对无线电大气掩星的影响,通过计算不同高度区间内的弯曲角偏差标准差统计研究电离层Es层及F层不均匀体发生率与大气弯曲角振荡的关系.结果表明,Es层及F层不均匀体能够引起大气弯曲角振荡,表现出弯曲角随纬度、磁地方时及季节变化的特性,且这些电离层小尺度因素能够影响到的高度区间为35～80 km.该研究有助于进一步认识电离层小尺度因素对GNSS无线电大气掩星探测技术的影响,以及后续提高大气掩星产品的数据质量.

The theoretical spectrum of incoherent scattering is the theoretical basis for understanding the detection of incoherent scatter radar. The solution process can be summarized into the microscopic method based on the work of Hagfors (1961) and the macroscopic method based on the framework of Farley (1960). In this paper, the above two methods are used to solve the theoretical spectrum of collisional non-magnetized plasmas, separately. The normalized admittance and polarizability of plasma and the differential scattering cross section and spectral density function of incoherent scattering are analyzed, and the mathematical relationships between them are obtained. A general analysis of the similarities and differences between the two methods in the theoretical choice, the final conclusion and a variety of factors are given.

In this paper, an efficient ensemble Kalman filter (EnKF) algorithm and the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model (NCAR-TIEGCM) are used to develop the ensemble Kalman filter data assimilation system. Based on the realistic observational configurations of space-based and ground-based global navigation satellite system (GNSS) ionospheric slant total electron content (TEC) observations and Challenging Minisatellite Payload (CHAMP) and Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (TIMED/GUVI) thermosphere measurements, we designed a series of observing system simulation experiments (OSSEs) to evaluate the performance of the system. We found that : (1) The parameters of the thermosphere can be optimized by assimilating ionospheric slant TEC via EnKF algorithm. (2) The performance of neutral mass density optimization is substantial in the whole assimilation stage, and the percentage of improvement can be up to 40%. (3) The integrated O/N-2 ratio (Sigma[O/N-2]) can be also optimized well during the assimilation period, but the effect becomes worse in the region where the horizontal gradient of electron density changes dramatically. Finally, the prediction of neutral mass density is evaluated. The results show that the prediction time scale can be up to 24 hours under the condition of geomagnetic quiet due to the optimization of neutral compositions.

Based on the international reference ionosphere (IRI) model, digital ionosonde and GNSS TEC data, we proposed a method which uses the empirical orthogonal function to estimate the topside ionospheric electron density profile and applied it to the Millstone Hill station. We then compared the estimated critical frequency and peak height with the digital ionosonde observations, and compared the estimated electron density above 400 km with density observed by incoherent scatter radar (ISR). Statistical results show that the estimated critical frequency and peak height are in consistent with the digital ionosonde data, the absolute error between modeled and ISR measured density above 400 km reduces 50% in comparison with that between digital ionosonde derivations and ISR. In summary, adding GNSS TEC in addition to digisonde could improve the topside ionospheric electron density estimation by our method accurately.

VLF (very low frequency) electromagnetic waves at 3 similar to 30 kHz have the characteristics of long wavelength and long propagation distance. They can propagate along the Earth-lower ionosphere waveguide, and are widely used in many fields including communication and navigation. The Long Wavelength Propagation Capability (LWPC) model based on the waveguide mode theory provides a useful tool to evaluate the propagation path and amplitude of VLF waves, which can be analyzed to investigate ionospheric disturbances caused by solar flares, magnetic storms, earthquakes and other extreme events. In this paper, the very simple electron density and collision frequency modules originally embedded in LWPC are updated by the International Reference Ionosphere (IRI) model for simulation improvements. The obtained numerical results are then compared to the observed amplitude of NWC VLF transmitter signals by Wuhan University VLF receiver at the Wuhan station. It is found that the amplitude variations of NWC VLF transmitter signals modeled using the LWPC and IRI models are much closer to the observations, which mainly results from the improved nighttime electron density profile from the IRI model and justifies the importance of electron density of the lower ionosphere to the VLF signal propagation properties. In addition, the dawn-dusk electron density variation on the wave propagation path largely modulates the NWC VLF signal amplitude, and forms an obvious transition region during the sunrise and sunset periods. Therefore, incorporation of the IRI model into LWPC improves quantitative analyses and prediction performance of the propagation processes of VLF transmitter signals, and provides an evaluation method of long wave navigation and communication quality.

It has been 60 years since the space physics as new branch of geophysics started to grow in 1957 when the space age was opened by a small satellite called sputnik.The knowledge of Earth and planetary space has been significantly extended and deepened,but the questions we are facing today are more challenging.A consensus reached is that we have to regard the Earth (planet) as an integrated system including all spheres from the inner core to the magnetosphere,and we should try to investigate some questions standing on the ground of interdisciplinary study,especially those questions related to Earth's (planetary) evolution.Space environment as the outer part of a planetary system,commonly exists in all planets but also exhibits strong diversity.Here,we introduce the short history of basic ideas and methods of comparative study,-the advantages on understanding of some issues of global scale,and the prospect from comparative perspective.

TIME-IGGCAS(Theoretical Ionospheric Model of the Earth in Institute of Geology and Geophysics,Chinese Academy of Sciences)模式是我们在前人工作的基础上完善的一个中低纬理论电离层模式.本文把该模式的结果与其他一些有代表性的经验模式和多种观测数据作了详细的对比.比较结果表明:TIME-IGGCAS模式模拟的电子浓度、电子离子温度在数量级上均与经验模型和观测符合得较好,在地方时变化、纬度变化、季节变化这些变化形态上也符合得较好,并且模式能很好地模拟出赤道异常、冬季异常和半年异常这些电离层异常,这为我们进一步开发电离层数据同化模式奠定了良好的基础.无论是与经验模式还是与观测相比,TIME-IGGCAS模式均低估了电子温度而高估了离子温度,模式在日出日落时段和在低高度模拟的偏差较大,这些结果为我们以后进一步完善模式、改善模式的模拟能力提供了参考.

在前人工作的基础上, 完善了一个中低纬电离层理论模式(TIME-IGGCAS: Theoretical Ionospheric Model of the Earth in Institute of Geology and Geophysics, Chinese Academy of Sciences). TIME-IGGCAS模式自洽的求解等离子体连续性方程、动量方程、能量方程, 对地磁场采用偏心偶极近似. 同时结合欧拉网格和拉格朗日网格, 并考虑了等离子体的电场漂移作用. 计算结果表明模式稳定可靠, 能再现大部分电离层大尺度特征. 基于本模式, 进行了一个观测系统数据同化试验. 结果表明, 采用非线性最小二乘拟合的方法, 把观测到的电离层临界频率和峰值高度同化到理论模式中, 能够准确的估算电场漂移作用, 为进一步开发同化模式, 进行电离层现报和预报奠定了基础.